Solid composite propellants with very high burning rates

ABSTRACT

A fast burning composite propellant comprising a cured intimate mixture of a prepolymer/plasticizer solution wherein the prepolymer is a copolymer of carboranylmethyl acrylate and acrylic acid and the plasticizer is selected from triethylene glycol dinitrate and butane-triol trinitrate and mixtures thereof, an inorganic oxidizing salt, and an additive of metal powder.

States Patent 1 aldwin et al.

[ Feb. 12, 1974 SOLID COMPOSITE PROPELLANTS WITH VERY HIGH BURNING RATES [75] Inventors: Mart G. Baldwin, Newtown; Lew R.

Beason, lvyland, both of Pa.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC

[22] Filed: Apr. 12, 1972 [21] Appl. No.: 243,430

Related US. Application Data [63] Continuation-impart of Ser. No. 836,176, June 24 1969, abandoned.

[52] US. Cl 149/19, 149/20, 149/21, 149/22, 149/113, 149/114 [51] Int. Cl C06d 5/06 [58] Field of Search... 149/19, 44, 20, 22, 113, 114, 149/21 Primary ExaminerBenjamin R. Padgett Attorney, Agent, or Firm-Edward J. Kelly; Herbert Ber]; Jack W. Voight ABSTRACT A fast burning composite propellant comprising a cured intimate mixture of a prepolymer/plasticizer solution wherein the prepolymer is a copolymer of carboranylmethyl acrylate and acrylic acid and the plasticizer is selected from triethylene glycol dinitrate and butane-triol trinitrate and mixtures thereof, an inorganic oxidizing salt, and an additive of metal powder.

4 Claims, No Drawings SOLID COMPOSITE PROPELLANTS WITH VERY HIGH BURNING RATES CROSS REFERENCE TO RELATED APPLICATION This is a continuation-in-part of application Ser. No. 836,176, filed June 24, 1969, now abandoned.

BACKGROUND OF THE INVENTION Fast burning composite propellants containing ammonium perchlorate and aluminum have been produced with burning rates in the range of 3-4 in./sec. at 2,000 psi. The high burning rates are achieved with propellants having a high solids loading (83-86%) of ammoniumperchlorate oxidizer (70-81%) and aluminum (5l6%) plus (8%) ballistic modifier. The ballistic modifiers frequently employed are iron oxide, ferrocene and n-butyl ferrocene. The propellants are difficult to prepare using conventional processing techniques because of their very high viscosity resulting from the high solids loading that is required to obtain high burning rates. In addition, the ballistic modifiers ferrocene and n-butyl ferrocene are somewhat volatile and tend to escape from the propellant during processing at elevated temperature and also under storage conditions at ambient temperature.

More recently, compounds such as dicarbaclovododecaborane (carborane) and derivatives of carborane have been employed as effective ballistic modifiers for obtaining high burning rates.

There exists a need for improved solid composite propellants with high burning rates. Improved solid composite propellants should possess a burning rate in the range of 24 in./sec., be relatively easy to process, and should contain ingredients with low volatility which are stable and do not escape from the propellant mixtures during processing at elevated temperatures and during storage at ambient temperature.

The principal object of this invention is to provide improved solid composite propellant compositions with high burning rates and qualitatively attractive mechanical properties.

A further object of this invention is to provide solid composite propellants which are relatively easy to process and which contain ingredients having low volatility.

A particular object of this invention is to provide solid composite propellants which employ a binder having a ballistic modifier as an integral part thereof for obtaining high burning rates.

SUMMARY OF THE INVENTION Acrylic acid (AA) copolymerized with carboranylmethyl acrylate (CMA) forms a prepolymer which is mixed with a plasticizer (e.g., triethyleneglycol dinitrate (TEGDN), or butanetriol trinitrate (BTTN) or mixtures thereof) to yield a prepolymer/plasticizer viscous fluid which is combined with a curing catalyst, inorganic oxidizing salt, and a powdered metal fuel to provide after curing a rubbery solid composite propellant with qualitatively attractive mechanical properties.

PREPARATION OF PREPOLYMER A low molecular weight copolymer of carboranylmethyl acrylate (CMA) and acrylic acid (AA) is prepared by polymerizing 95 parts of CMA with 5 parts of AA. The polymerization reaction is carried out in ethyl acetate solvent (solvent/monomer 5/ l at about C. with benzoyl peroxide initiator (l gram/100 grams monomer). After the reaction is completed (in approximately 16 hours) and the solution allowed to cool to ambient temperature, a non-volatile, compatible plasticizer is added to the solution prior to removing the solvent at reduced pressure. After removal of the solvent the product is a homogeneous, viscous fluid consisting of a low molecular weight copolymer of CMA and AA dissolved in the plasticizer of choice.

Since acrylic polymers have inherently good compatibility with many common plasticizers, a wide variety of plasticizers can be used to formulate propellant compositions for a number of specific end uses. Triethyleneglycol dinitrate and butanetriol trinitrate in about equal parts make a plasticizer mixture useful in this invention. Also, the plasticizer may be separately used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Two fast burning propellant formulations are given below as Example I, Composition A, and Example II, Composition B. Table I sets forth burning rate, in./sec. when the specified particle-sized ammonium perchlorate is used. The table provides data whereby burning rates may be interpolated for various particle sized ammonium perchlorate used in accordance with Composition A.

Carboranylmethyl acrylate] Acrylic acid (/5) prepolymer Triethyleneglycol dinitrate 10.8 Butanetriol trinitrate (contains in addition 2 parts of dicyclo diepoxy carboxylate per I00 parts of propellant) Also known as l,2,4-trinatrobutane 10.9

Dicyclo diepoxy carboxylate is a curing agent. A dicyclo diepoxy carboxylate curing agent is sold under Union Carbide Corporations trademark UNOX 221.

TABLE I PARTICLE SIZE OF AMMONIUM PERCHLORATE' AND BURNING RATE AT 2000 PSI Particle Size (Normal) Burning Rate, in.lsec. 75 micron 2.0 45 micron 2.2 35 micron 2.4 15 micron 3.0 5 micron 3.6

The burning rates depicted in Table I indicate the range of burning rate attainable with the propellant composition using carboranylmethyl acrylate and acrylic acid (95/5) prepolymer along with the other components specified elsewhere in this document.

Although composition B has an average ammonium perchlorate particle size of 15 microns the propellant has a burning rate of 3.65 in./sec. at 2,000 psi. The processibility of compositions A and B is good. The good processibility of both compositions A and B is largely due to the relatively low total solids loading (60-61%) as compared to the high solids loading (83-86%) required to obtain burning rates in the 3-4 in./sec. range for other composite propellants described under prior art.

Inorganic oxidizing salts in amounts of about 35 to about 60 weight percent are used in the propellant compositions of this invention. Although ammonium perchlorate is the preferred inorganic oxidizing salt for use with this invention, any of the inorganic oxidizing salts such as ammonium, alkali metal, and alkaline earth metal salts of nitric, perchloric, and chloric acids and mixtures thereof are suitable oxidizing salts for use in this invention. Powdered metals (as fuels) such as aluminum, magnesium, titanium, zirconium, and boron are suitable for use in the propellant composition in amounts of from about 5 to about 20 weight percent. Aluminum is the preferred metal. Plasticizers such as the esters of phthalate, adipate, sebacate, and aryl phosphate may be used. Triethyleneglycol dinitrate and butanetriol trinitrate are preferred plasticizers. Acrylic polymers have inherently good compatibility with many common plasticizers, therefore, a wide variety of plasticizers can be used to formulate propellant compositions for a number of specific end uses. Plasticizers such as triacetin and adiponitrile may also be used for certain propellant formulations. An amount of plasticizers from about to about 30 weight percent is generally needed to obtain desired properties in propellant during processing and in the finished propellant.

The prepolymer mix of 95/5 carboranylmethyl acrylate (CMA) and acrylic acid (AA), may vary in the propellant composition from about 12 to about 25 weight percent. Also, the ratio of CMA to AA in the prepolymer may vary from about 90/ 10 to about 96/4; however, the 95/5 mix is the preferred mix for use with the plasticizers specified.

Since CMA is an effective burning rate promoter, and the same is copolymerized with acrylic acid to form the useful prepolymer which is later mixed with the other propellant ingredients and cured to form the binder, no separation of the burning rate promoter or catalyst is encountered. Migration after casting or separation during mixing has been a problem of prior art catalysts utilization in propellants. The novel prepolymer composition containing the burning rate catalyst as an integral part does provide a very useful and convenient to use material for compounding precise propellant compositions having controllable burning rates and qualitatively attractive mechanical properties.

In addition to the major components, the propellant compositions of this invention may contain trace amounts of curing agents, processing aids (e.g., lecithin), stabilizers, and ballistic modifiers as required for specific conditions.

The dicyclo diepoxy carboxylate curing agent used in compositions A and B is more extensively identified under UNOX 221 in The Condensed Chemical Dietionary, Seventh Edition, Copyright 1966 by Reinhold Publishing Corporation. The dicyclo diepoxy carboxylate which is sold under Union Carbide Corporations trademark UNOX 221 has the following properties: Sp. gr. 1.1750 (20/20 C); b.p. 354 C (760mm); vapor pressure 0.01 mm hg. at 20 C; f.p. 20 C (sets to glass below this temperature); at 20 C, 0.03% by wt. soluble in water; water in, 2.8% by wt; wt/gal 9.76 at 20 C; flash point 400 F.

The disproportionation of two molecules of an aldehyde (usually aromatic) brought about by the action of sodium or potassium hydroxide to yield the corresponding alcohol and acid is known as the Cannizzaro reaction. A review of the Cannizzaro reaction is given by T. A. Geissman, in Organic Reactions (R Adams, ed.), Vol. II, p. 94. Wiley, New York, 1944.

One method of synthesis to produce a dicyclo diepoxy carboxylate curing agent of the type sold under trademark UNOX 221 employs a Cannizzaro-type redox reaction which is a reaction between an aldehyde an alcoholate. The reaction is summarized by the chemical equation which follows the Procedure for the Synthesis of 4,5Epoxycyclohexyl-methyl 4',5'- epoxycyclohexylcarboxylate set forth hereinbelow.

PROCEDURE FOR THE SYNTHESIS OF 4,5EPOXYCYCLOHEXYLMETHYL 4'5'-EPOXYCYCLOHEXYLCARBOXYLATE Metallic sodium (3g., 0.13 g-atom) is dissolved by warming for 7% hour in 4,5epoxycyclohexylmethyl alcohol (82g. 0.65 mole). After the mixture has cooled to room temperature, the solution is added gradually, with thorough mixing, to 4,5epoxycyclohexaldehyde (529g, 4.3 moles)(which contains less than 1% of 4,5epoxycyclohexycarboxylic acid). The reaction mixture has a tendency to become warm, but the temperature should be maintained somewhat below -60, by cooling, if necessary. A pasty, gelatinous mass results. After about '26 hour, the temperature of the mixture no longer rises; it isthen warmed on a water bath for l-2 hours with occasional shaking.

The cooled reaction product is treated with water (200 ml.); the layer of oil is separated; washed once with a second portion of water, and subjected to distillation under reduced pressure. The first fraction of the distillate contains recovered epoxyeyclohexylmethyl alcohol together with unchanged epoxycyclohexaldehyde, as well as a small quantity of water. The temperature then rises rapidly to the boiling point of the epoxycyclohexylmethyl epoxycyclohexylcarboxylate. The product boils at 184-185/5mm or in the range of about 350-360 C/760mm. The yield is 4l0-420g (-80% of the theoretical amount).

The resulting product has a viscosity of 350-450 centipoise at 25 C; a specific gravity of 1.16-1.18; its color on the Gardner Color Comparator is less than 1.

The method of synthesis is by a Cannizzaro-type redox reaction, as summarized by the following chemical equation:

CHr-OzC- *The S in the hexagon means cyclohexane.

We claim:

1. Solid composite propellant compositions characterized by their very high burning rates and comprising a cured mixture of an inorganic oxidizing salt, plasticizers that are compatible with the other propellant ingredients, metal additives, dicyclo diepoxy carboxylate curing agent, ferric acetylacetonate curing agent, and a copolymer constituted substantially of carboranylmethyl acrylate and acrylic acid, said copolymer being the prepolymer product formed by polymerizing the monomer, carboranylmethyl acrylate, with the monomer, acrylic acid in a ratio of parts carboranylmethyl acrylate to acrylic acid from about 90 to to about 96 to 4 in the presence of ethyl acetate solvent present in the ratio of (solvent to monomer of about 5 to 1) and employing benzoyl peroxide initiator at the rate of 1 gram/100 grams monomer.

2. The propellant composition of claim 1 and wherein said inorganic oxidizing salt is selected from ammonium, alkali metal, and alkaline earth metal salts of nitric, perchloric, and chloric acids, and mixtures thereof in an amount from about 35 to about 60 weight percent; said plasticizers are selected from triethyleneglycol dinitrate, butanetriol trinitrate, triacetin, and adiponitrile in an amount from about 10 to about 30 weight percent; and metal additive is selected from aluminum, magnesium, titanium, zirconium, and boron in amount from about 5 to about weight percent;

said dicyclo diepoxy carboxylate curing agent is present in an amount from about 1 to about 4 weight percent; and wherein said copolymer is present in amount from about 12 to about 25 weight percent.

3. The propellant composition of claim 2 and wherein said inorganic oxidizing salt is ammonium perchlorate present in amount of about 55.0 weight percent; said metal additive is aluminum present in amount of about 5 weight percent; said copolymer, constituted of carboranylmethyl acrylate about parts and acrylic acid about 5 parts, is present in amount of about 22.06 weight percent; said dicyclo diepoxy carboxylate curing agent is present in amount of about 3.09 weight percent; said plasticizer is triethyleneglycol dinitrate present in amount of about 14.70 weight percent; and wherein said ferric acetylac' etonate curing agent is present in amount of about 0.15 weight percent.

4. The propellant composition of claim 2 and wherein said inorganic oxidizing salt is ammonium perchlorate, blended of about three parts of said ammonium perchlorate having a nominal particle size of about 5 microns and one part of said ammonium perchlorate having a nominal particle size of about 45 microns, present in amount of about 56.0 weight percent; said copolymer, constituted of carboranylmethyl acrylate about 95 parts and acrylic acid about 5 parts, is present in amount of about 16.3 weight percent; said plasticizer is comprised of triethyleneglycol dinitrate in amount of about 10.8 weight percent and butanetriol trinitrate in amount of about 10.9 weight percent; said metal additive is aluminum present in amount of about 5 weight percent; and wherein said dicyclo diepoxy carboxylate curing agent is present in amount of two parts to parts of the remaining propellant ingredients. 

2. The propellant composition of claim 1 and wherein said inorganic oxidizing salt is selected from ammonium, alkali metal, and alkaline earth metal salts of nitric, perchloric, and chloric acids, and mixtures thereof in an amount from about 35 to about 60 weight percent; said plasticizers are selected from triethyleneglycol dinitrate, butanetriol trinitrate, triacetin, and adiponitrile in an amount from about 10 to about 30 weight percent; and metal additive is selected from aluminum, magnesium, titanium, zirconium, and boron in amount from about 5 to about 20 weight percent; said dicyclo diepoxy carboxylate curing agent is present in an amount from about 1 to about 4 weight percent; and wherein said copolymer is present in amount from about 12 to about 25 weight percent.
 3. The propellant composition of claim 2 and wherein said inorganic oxidizing salt is ammonium perchlorate present in amount of about 55.0 weight percent; said metal additive is aluminum present in amount of about 5 WEIGHT percent; said copolymer, constituted of carboranylmethyl acrylate about 95 parts and acrylic acid about 5 parts, is present in amount of about 22.06 weight percent; said dicyclo diepoxy carboxylate curing agent is present in amount of about 3.09 weight percent; said plasticizer is triethyleneglycol dinitrate present in amount of about 14.70 weight percent; and wherein said ferric acetylacetonate curing agent is present in amount of about 0.15 weight percent.
 4. The propellant composition of claim 2 and wherein said inorganic oxidizing salt is ammonium perchlorate, blended of about three parts of said ammonium perchlorate having a nominal particle size of about 5 microns and one part of said ammonium perchlorate having a nominal particle size of about 45 microns, present in amount of about 56.0 weight percent; said copolymer, constituted of carboranylmethyl acrylate about 95 parts and acrylic acid about 5 parts, is present in amount of about 16.3 weight percent; said plasticizer is comprised of triethyleneglycol dinitrate in amount of about 10.8 weight percent and butanetriol trinitrate in amount of about 10.9 weight percent; said metal additive is aluminum present in amount of about 5 weight percent; and wherein said dicyclo diepoxy carboxylate curing agent is present in amount of two parts to 100 parts of the remaining propellant ingredients. 